How to fix render darken after texturing in blender cycles?
An expanded answer of @cegaton and @PCmath.

In cycles lighting can be one of the most important aspects of a render, it can make a dull image into a full vibrant master piece.

Here's a quick render with only three objects: A floor, Suzanne, and finally a camera. I added no lights to this scene. 



Here's the exact same scene with a light, i used the light "Sun". 



Without lights the scene can look dull. To add a light press the tab on your bottom left labeled "Add" click expand lights, and select your desired light. Experiment and find out what's best for your scene.